First, I would keep her inactive as possible (i.e.limit her activity...no running jumping etc.). Try to palpate her knee and she is she shows any signs of distress. What you are describing is exactly what Jewel had...torn ACL. You need to see your vet ASAP. If Ms. Charlotte has a positive "Drawer Sign," then it is definitely an AL issue. X-rays will not pick-up a torn ACL because the ACL is soft tissue but then again there maybe a fracture which in most cases will show on an x-ray. Fingers, toes, and paws are crossed that it is nothing serious as I have mentioned.